influence of oxidation of automatic transmission fluids
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Friction 9(2): 401–414 (2021) ISSN 2223-7690 https://doi.org/10.1007/s40544-020-0406-z CN 10-1237/TH
RESEARCH ARTICLE
Influence of oxidation of automatic transmission fluids (ATFs) and sliding distance on friction coefficients of a wet clutch in the running-in stage
Leonardo Israel FARFAN-CABRERA1,*, Ezequiel Alberto GALLARDO-HERNÁNDEZ2, Manuel VITE-TORRES2,
Jesús Gilberto GODÍNEZ-SALCEDO3 1 Tecnologico de Monterrey, Escuela de Ingeniería y Ciencias, Ave. Eugenio Garza Sada 2501, Monterrey 64849, México 2 Instituto Politécnico Nacional, SEPI-Escuela Superior de Ingeniería Mecánica y Eléctrica, Unidad Zacatenco, Grupo de
Tribología, Col. Lindavista, Ciudad de México 07738, México 3 Instituto Politécnico Nacional, ESIQIE, IPN, Col. Lindavista, Ciudad de México 07738, México
Received: 15 January 2019 / Revised: 06 July 2019 / Accepted: 18 May 2020
© The author(s) 2020.
Abstract: In this paper, the influence of oxidation of automatic transmission fluids (ATFs) and sliding
distance on the friction coefficients of a wet clutch in approached running-in conditions was investigated.
The ATFs were oxidized by a laboratory process approaching oxidation occurred in actual ATFs. Oxidation
was evaluated by means of increase in carbonyl compounds and depletion of zinc dialkyldithiophosphates
(ZDDPs) additives. Also, the changes in kinematic viscosity and viscosity index were evaluated. Pin-on-
disk tests were conducted to replicate the actual sliding contact in a wet clutch. The pin specimens were
cut from friction material composite plates and the disks were actual steel separators both from an
automotive wet clutch. Friction coefficient, μ, was measured at progressive sliding velocity, ν, to obtain
μ–ν curves at 26 and 100 °C. Three μ–ν tests were consecutively run using the same pair of specimens and
oil. The cumulative sliding distance for each μ–ν test generated surface flattening using the oils. The
friction coefficients of the wet clutch increased due to the ATFs oxidation meanwhile the d/dv values
decreased in most cases. It suggests that ATF oxidation can enhance torque capacity of the wet clutch, but
it could reduce anti-shudder property. Progressive sliding distance improved the slopes in the μ–ν results
using fresh ATFs meanwhile it generated a slope decrease by using aged ATFs.
Keywords: wet clutch; friction; oil oxidation; automatic transmission fluid (ATF)
1 Introduction
Wet clutches are components widely used for
power shifting, lock-up, and continuously slipping
to transmit torque in automatic transmissions (ATs)
from cars. In a clutch engagement, as load is applied
to press the set of plates with relative rotating
motion, the automatic transmission fluid (ATF)
film at the plates is squeezed, so sliding contact of
the opposing surfaces occurs to transmit torque.
Torque transmission across the disk interface is
due to viscous shearing of the ATF and asperities
contact between the surfaces of the plates. Low
tribological performance either from the ATF or
plates produces poor torque transmission, which
leads to increased fuel consumption, vehicle vibration,
excessive wear of plates, and audible noise (squeal,
chatter, and shudder) [1]. Therefore, an ATF must
meet different requirements as hydraulic fluid
and lubricant for the good performance of an AT.
* Corresponding author: Leonardo Israel FARFAN-CABRERA, E-mail: farfanl@hotmail.com
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For example, it must meet with low viscosity,
high viscosity index, good lubricity, anti-wear, anti-
oxidation, and anti-shudder properties, etc. [2].
In order to meet these requirements, an ATF is
formulated with a mineral or synthetic base oils
and different additives such as anti-wear agents, anti-
oxidants, detergents‒dispersants, and organic friction
modifiers. The additives can interact individually or
synergistically to contribute to its good performance,
in particular, to the frictional characteristics of
the wet clutch. Particularly, a wet clutch system is
required to operate with high friction to reduce clutch
engagement times and allow torque transmission,
so both together ATF and clutch plates should
generate these lubrication characteristics.
The progressive sliding of wet clutch plates produces
alteration to the surface of the plates, which is
majorly caused to the friction composite material
plate’s surface. It initiates with the running-in
stage, in which both steel separator and friction
disk surfaces are flattened by plastic deformation
of metallic asperities and collapse of friction material
composite pores reducing surface porosity. Afterwards,
a darkened and either a smooth or shiny surface in
the friction material disk is produced, which is
ascribed to the formation of a thin smooth and
shiny coating made of degraded products from the
ATF on the pore structure by lodging effect. This is
commonly known as “glazing” [3]. As glaze layer
magnitude increases, surface porosity also decreases.
The decrease in surface porosity limits ATF diffusion
and permeability in the friction material as required
in the ideal engagement condition to allow ATF
squeezing through the engagements. Advanced
glazing is very well identified by a thick shiny layer
on the friction material surface, which rises temperature
and wear, and reduces torque transmission capacity
due to lower friction between the plates [3]. The
continuous sliding, ATF degradation, surface flattening,
and glazing of the friction plates may generate
reduction of torque transmission capacity, but also
undesirable self-excited vibrations by stick-slip
effects, which is named as “shudder” [4]. It has
demonstrated that shudder may occur when friction
coefficient, , in the clutch plates decreases as the
sliding speed, ν, increases, so a positive slope,
d/dv, therefore needs to be maintained in curves
of friction coefficient versus sliding velocity (μ–ν)
to prevent shudder, in particular, in full scale tests
since it depends on many factors included in the
torque convertor operation [4–6].
Overall, since torque transmission performance
of the wet clutch system completely depends on
frictional performance, it is also indispensable that
the friction properties can be maintained in both
running-in and long-term use of wet clutch, especially
in continuously slipping clutches [5]. It is well known
that anti-shudder performance of an ATF in a wet
clutch system is progressively affected by the high
energy engagements and oxidation due to the AT´s
operation [7]. Watts et al. [8] evaluated the status
of anti-shudder property of field ATF samples from
actual lock-up clutches exhibiting shudder in an
adapted SAE (Society of Automotive Engineers)
#II Friction Test Machine to determine the cause of
the onset of vehicle shudder. In addition, low-
velocity friction apparatus (LVFA) tests were carried
out to complement the analyses. They were useful
to separate, in some extent, the contribution of
hardware of SAE #II machine to the loss of shudder
control. It was found that shudder was produced
due to rising static friction in the SAE #II tester.
Similarly, Li et al. [9] and Devlin et al. [6] studied
the effects of glazing of clutch plates and the ATF
ageing on the anti-shudder performance and torque
capacity in wet clutches using the SAE #II and
LVFA tests, respectively. In most the studied cases,
it was found that fluid ageing has larger influence
on the anti-shudder durability than surface glazing.
Although the SAE #II and LVFA tests have been
demonstrated as very suitable for the measurement
of friction characteristics of wet clutches, it has
been reported that the anti-shudder durability tests
give no-correlated results between each other [7],
which could be attributable to the influence of particular
conditions that cannot be accurately controlled in each
tester. In order to evaluate the friction characteristics
of wet clutch under more controlled conditions,
Berglund et al. [10] used a pin-on-disk test to
investigate the effects of ATF ageing and different
additive concentration on the friction characteristics
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of a wet clutch with plates made of sintered bronze
friction material. The ATF was oxidized in lab by
heating it up at 120 °C for five periods from 48 to
408 h by an oxidation stability test approaching
the ageing process of a wet clutch used in a field
test. Since the effect of surface alteration of plates on
the friction was not their research purpose, the
friction coefficients of new friction material composite
samples under increasing sliding speeds at 30,
70, and 100 °C were only assessed. In general,
they found that friction increased with the ATF
oxidation. In comparison to SAE #II, LVFA, and other
complex tests, pin-on-disk tests were found to be
very inexpensive and time saving for large series of
wet clutch testing allowing evaluations of many
different combinations, including materials, ATFs,
and test conditions. Moreover, it allowed an easy and
accurate control of test parameters and measurement
of more local friction [11]. It was also found to be a
suitable and alternative option to obtain measurements
of friction coefficient versus sliding speed using
ATFs and friction material composites for wet
clutch [10, 12]. The results of friction coefficients at
different temperatures, contact pressures, and
sliding speeds from this method have been found
to correlate well with results obtained from a SAE
#II machine [11, 13].
Although there are some reported attempts to
recognize the effects of ATF oxidation and surface
damage of plates on the frictional behavior of wet
clutches, it is not well understood yet, especially in
the running-in stage. Measurements of local friction
of wet clutch plates under controlled sliding distance,
temperature, speed, load, and ATF condition discarding
other effects can be helpful for achieving a better
understanding.
Hence, this research paper aims to contribute
with an evaluation of the influence of oxidation of
two commercial ATFs and the progression of surface
alteration of friction material composite plates
caused by progressive sliding distances on the
friction coefficients of a wet clutch. The tests were
carried out in a pin-on-disk tester at increasing
speeds approaching the wet clutch sliding interface
in the running-in stage.
2 Material and methods
2.1 Oil samples
The lubricants tested were two common ATFs with
different specifications, which will be named in
the following as: 1ATF and 2ATF. 1ATF meets the
specification for DEXRON III, Ford MERCON,
and Allison C-4/C-3 meanwhile 2ATF meets only
the specification for MERCON V according to the
oil data sheets, respectively. They were selected to
evaluate two oils meeting different ATF specifications.
The oils were exposed to a laboratory thermal
ageing process [14] which was developed based on
Ref. [15] in order to approach oxidation occurred
in actual ATFs in automotive transmissions with
the long-term use. The oil oxidation produced in
laboratory has been demonstrated as a useful tool
to study specific degradation characteristics of oils
under controlled conditions [16]. The oil ageing
set-up can be seen in Fig. 1. This method has been
demonstrated as a pronounced short-term method
to investigate the long-term behavior of automotive
oils enabling the pre-selection thereof according to
their performance under controlled conditions. It
consisted on oxidizing an oil sample of 300 g in a
sealed glass vessel at 160 °C for 60 h by using a
stirring hot plate with temperature control with
±0.5 °C of accuracy. The temperature selected for
the oil ageing corresponds to an average of the
highest temperatures occurred at the interface of
wet clutch plates during engagement [17, 18]. Two
conduits made of glass for air inlet and exhaust
gases were adapted to the vessel. The air was
introduced in the oil through the air inlet conduit
in order to facilitate the oxidation process and
agitate the sample during the test. The air flow
was adjusted at 10 L/h as suggested in the method.
The relative air humidity was measured to be in
the range of 40%–50%. Also, a thermometer was
immersed in the oil to measure temperature. Although
this method has demonstrated an acceptable correlation
with mineral oils used in actual operating conditions,
there are not specific information about the correlation
of oxidation of ATFs in lab with those oxidized in
actual ATs. Thus, to have a reference of the oxidation
caused in lab by this method, one of the oils (2ATF)
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Fig. 1 Schematic view of the laboratory thermal ageing set-up.
was additionally aged in a field test to be compared
with the 2ATF oxidized in lab. For this, the oil was
used in a new car equipped with an AT with torque
converter. After a use of 50,000 km, considered as a
substantial use period for an ATF in a passenger
car, the ATF was extracted to analyze the oxidation
produced.
The standard method ASTM D-7214 was carried
out to evaluate oxidation caused in the ATFs. It
consisted on determining oxidation of oils by Fourier
transform infrared (FTIR) analysis by measuring
the change in concentration of constituents containing
a carbonyl function that have formed during the
oxidation process. FTIR spectra of the fresh and
aged oil are recorded in a transmission cell of known
pathlength. Both spectra are converted to absorbance
and subtracted. Using the differential spectrum, a
baseline is set under the peak corresponding to the
carbonyl region between 1,650 and 1,820 cm‒1. The
area created by this baseline and the carbonyl peak
is calculated and then divided by the cell pathlength.
The result is reported as peak area increase (PAI).
Also, the standard method ASTM D-7412 was used
to monitor the depletion of zinc dialkyldithiophosphates
(ZDDPs) additives, which are contained in the ATFs
tested according to the datasheet. The specific
concentration of ZDDPs was not provided by the
manufacturer. However, the standard method based
on FTIR analyses helped to visualize the change in
absorbance corresponding to ZDDPs in function of
thermal ageing. These additives are the most used
in the formulation of mineral and synthetic ATFs to
reduce friction and wear of metallic components, but
mainly to prolong oil oxidation resistance. The
content of additives of the oil samples was estimated
from each FTIR spectrum by using the direct trend
analysis measurement in the area between 960 and
1,025 cm‒1 and the minimum and maximum baseline
points, 700 and 1,900 cm‒1, respectively. Both analyses
were performed using a spectrometer micro Raman
coupled to an ATR (objective 36×) from a module
FTIR (IR2). The cell pathlength was set at 0.05 mm.
The spectral data was collected between 700 and
1,900 cm‒1 averaging 32 scans with a resolution of
4 cm‒1. The data range was selected accordingly to
meet the required spectra for analysis in both
standard methods.
In addition, the viscosity changes occurred by
thermal ageing were evaluated. Dynamic viscosities
were measured by using a rotational rheometer
with a cone/plate configuration at 26, 40, and 100 °C.
The kinematic viscosities were calculated by considering
the dynamic viscosities and densities obtained. Also,
the viscosity indexes were estimated by the ASTM
D-2270 standard procedure in which the kinematic
viscosities at 40 and 100°C were considered. All
the tests mentioned above were carried out for the
fresh and aged samples.
2.2 Friction test
The friction tests were carried out using a test
configuration in a pin-on-disk tester under specific
controlled conditions to approach the sliding contact
of an actual wet clutch. The schematic view of the
test set-up used can be seen in Fig. 2. Constant
normal loads, oil temperatures, and rotational speeds
could be evaluated in the tester. The pin and disk
specimens and the corresponding holders are
illustrated in Fig. 3. The pin and disk samples were
prepared from the selected wet clutch plates. The
friction disks corresponded to plates of resin- filled
cellulose (friction material composite) bonded to a
steel backing plate. This class of material is used in
the market for common wet clutches from
powershift transmissions in passenger cars [19].
The disk samples were new steel plates made of
high carbon steel with material specification CS 70
according to the manufacturer data sheet. They
have a thickness of 1.9 mm, hardness of 255 HB,
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and surface roughness (Ra) of 0.22 ± 0.03 μm. The
actual housing of the wet clutch was modified and
adapted to operate as the disk holder and oil
container to secure the disk (steel plate) in the
proper position for the test and to contain the oil
to be tested. The holder for the pin specimen was
manufactured in order to secure small rounded
specimens of 9 mm in diameter. For the test, the
pin specimen was positioned at the end of the pin
holder in a small cavity. A small spherical setting
arrangement in the lower part of the pin holder
that moves about a central point was manufactured
to allow alignment of the flat pin sample to the
disk surface. The load was axially applied via dead
weights to the friction material composite sample
being in full contact with the steel disk surface.
The pin on disk contact was entirely immersed in
lubricant during the whole test using 100 mL of oil.
The friction force generated by the sliding between
the pin and disk specimens was measured and logged
by using a load sensor and a data acquisition
software. The friction coefficients were evaluated
at different sliding speeds (μ–ν test). The test conditions
can be seen in Table 1. The tests started with a
velocity of 0.05 ± 0.01 m/s, which was increased in
increments of 0.06 ± 0.01 m/s up to 0.9 ± 0.01 m/s
Fig. 2 Schematic view of the pin-on-disk set-up.
Table 1 Friction test parameters.
Test parameter 1st Run 2nd Run 3rd Run
Load (N) 51 51 51 Contact pressure (MPa)
0.8 0.8 0.8
Oil temperature (°C)
26±1, 100±1 26±1, 100±1 26±1, 100±1
Sliding speed (m/s) 0.050.9 0.050.9 0.050.9
Sliding distance (m)
0–634±1 634±1–1269±1 1269±1–1904±1
using the same sample pairs. Although anti-shudder
standard tests are usually carried out at speeds
lower than 0.05 m/s, the dynamic friction coefficient
could be measured till a minimum speed of 0.05 m/s
due to the capabilities of the pin-on-disk tester used.
The friction force was measured for each speed
interval for a period of 90 s, collecting 10 data/s. It
was selected since the standard deviation of friction
coefficient for this period was found to be lower
than 0.015 ± 0.001 in previous trials, which suggested
friction coefficient stability by using this measurement
period. In total, for each μ–ν test, 15 different velocities
were tested, completing a sliding distance of 634 m.
Since the surface of friction material composite
plates is altered by the progressive sliding distance,
the same pair of specimens and oil sample were used
again to conduct other two tests consecutively under
similar conditions to examine different levels of
surface alteration. The surface alteration was assessed
in terms of changes in surface roughness (Sa) by
using an optical profilometer with an objective of 5×.
The friction coefficient at different sliding speeds
was evaluated for three consecutive sliding distances.
The first run was in the range from 0 to 634 m, the
second run was from 634 to 1,269 m, and the third
run was from 1,269 to 1,904 m. Since there are not
published results about the surface alteration of
friction disks under the certain friction test conditions
used for this work, several pre-tests were carried
out to achieve homogeneous and visible surface
alteration (flattening and darkening) on the samples
by generating stable friction coefficient through the
whole tests. A normal load of 51 N was used to
generate an apparent contact pressure of 0.8 MPa
to be in range of contact pressures (0–2.9 MPa)
generated in actual wet clutches from automatic
power shift transmission of moving off-road vehicles
[13, 20]. Each oil sample at each temperature was
tested by using a new pair of pin and disk specimens.
Three friction tests under similar conditions for
each oil and each temperature were performed.
The tests were run at two oil temperatures (26 and
100±1 °C) to replicate the operation temperature
of an automatic transmission under starting-up
and overheating conditions, respectively. Also, they
were selected due to the minimum and maximum
temperature control capabilities of the tester used.
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Fig. 3 (a) Pin specimen/friction material sample; (b) steel disk specimen and disk holder/oil container; and (c) representation of the pin-on-disk configuration used for the friction test.
3 Results and discussion
3.1 Effects of thermal ageing on ATFs
3.1.1 Oxidation
The FTIR spectra obtained from both fresh and aged
oils are shown in Figs. 4(a) and 4(b), respectively.
The spectrum ranges considered for the standard
measurements (ASTM D-7214 and ASTM D-7412)
are also indicated in these figures. In general, for
mineral oils, the oxidation is mainly represented
by increase in the area between 1,650 and 1,820 cm‒1,
which is known as the carbonyl region. The growth
of such an area is mainly caused by the absorbance
increase in the peak near to 1,728 cm‒1. Another
common effect of thermal ageing in ATFs is the
depletion of additives, in particular, ZDDPs. The
area created by the absorbance in the peak near to
968 cm‒1 represents the content of this kind of
additives in the sample according to that specified
in the standard method ASTM D-7412, so a decrease
of intensity of this area suggests ZDDPs depletion.
In the spectra, absorbance in the peaks near to 1,377
and 1,460 cm‒1 was also identified. The absorbance
of these peaks suggests the content of mineral oil
molecules with C–H bonds modes [21], which are
representative of the ATFs tested. In Fig. 4(b), the
spectra from 2ATF used in the field test are
additionally shown to be compared. The ageing
produced in 2ATF by the laboratory and field ageing
tests did not correlate closely since 2ATF aged in
the field test exhibited higher additives depletion
and higher increase in the carbonyl region than
those presented in 2ATF aged in lab. On the other
hand, comparing qualitatively both Figs. 4(a) and
4(b), 2ATF exhibited higher increase of absorbance
in the carbonyl region than 1ATF, which suggested
that it was more oxidized. Also, higher absorbance
of the peak near to 968 cm‒1 in both spectra from
2ATF can be seen, which suggested that this oil had
higher ZDDPs concentration than 1ATF. This can
be more clearly seen in Figs. 5(a) and 5(b), in which
a quantitative comparison of PAI value and ZDDPs
concentration obtained from the oils is shown,
respectively. Although 2ATF had higher ZDDPs
concentration than 1ATF, it was more affected by
the thermal ageing. It was perhaps due to a better
refining quality of its base oil or the content of
other additives incorporated in the formulation of
1ATF. Thus, it can be said that 1ATF was more
resistant to oxidation by thermal ageing than 2ATF.
2ATF used in the field test presented higher ZDDPs
depletion and oxidation than 2ATF aged by the
laboratory test, suggesting that the ageing characteristics
evaluated did not correlate exactly under those
conditions. So, more laboratory and field tests under
different conditions, chemical evaluations and testing
of other ATFs are needed to support and enhance
the technical accuracy of this oxidation method.
It is noteworthy that the ATFs ageing could be
also influenced by the humidity existing during
the aeration for the ageing process. This can produce
a rapid reverse of micelles formation and acceleration
of oil degradation as reported in other research
works [22, 23]. Another consequence is the detriment
of tribological performance of ZDDPs due to the
depolymerization of longer chain phosphates by
the oil contamination with water [24]. The oxidation
of mineral oils is mainly evaluated by means of
increase in carbonyl compounds and depletion of
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Fig. 4 FTIR spectra from the fresh and aged ATFs samples: (a) 1ATF; (b) 2ATF.
Fig. 5 (a) Comparison of oxidation (PAI values) caused in the ATFs; (b) comparison of ZDDP concentration.
anti-oxidant additives, such as ZDDPs. However,
ageing involves other changes in the base oil and
other chemical compounds (additives) contained
in an ATF, namely, friction modifiers, dispersants,
detergents, etc., which can also have an effect in
the friction coefficients of the wet clutch [22]. The
study of the effect of oxidation of each of the above
additives either independently or synergistically on
the wet clutch friction requires extensive further
work.
3.1.2 Changes in viscosity
The comparison of viscosities for the fresh and
aged ATFs at 26, 40, and 100 °C are shown in Figs.
6(a)–6(c), respectively. 1ATF exhibited a negligible
increase of viscosity (less than 1 cSt) at all the
temperatures tested after the ageing process while
2ATF presented a much higher viscosity increase.
It can be attributed to the increase of the carbonyl
region found in the spectra of this oil, as seen in
Fig. 5(b), since the carbonyl compounds induce
hydrogen bonding, dipole, and van der Waals
interactions among the constituents present in mineral
and synthetic oils and contribute to change viscosity
[25]. In general, the increase of viscosity in ATFs
can enormously compromise efficiency of an AT,
mainly at low temperatures, since the ATF fluidity
may be reduced producing deficiencies in the
clutch engagement either by its performance as
hydraulic fluid or as wet clutch lubricant. Moreover,
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Fig. 6 Comparison of kinematic viscosity of fresh and aged
ATFs at: (a) 26 °C; (b) 40 °C; and (c) 100 °C; (d) comparison
of viscosity index for the fresh and aged ATFs.
a high viscosity index is required for ATFs to limit
considerably the change of oil viscosity by temperature
effects. In Fig. 6(d), the comparison of viscosity
index of the fresh and aged oils is shown. In
general, the viscosity index from both ATFs
decreased with thermal ageing. However, 2ATF
exhibited the highest viscosity index before and
after the ageing process.
3.2 Friction coefficients
The comparisons of the friction coefficients against
sliding speed from three consecutive runs of fresh
and aged ATFs at 26 and 100 °C can be seen in Figs.
7 and 8, respectively. The friction coefficient results
represent the average of the data measured for 90 s
for each sliding speed from the three repetition
tests. The standard deviation found for friction
coefficient from the three repetitions at each speed
was lower than 0.016. In all the plots, the results of
friction coefficient exhibited increase or decrease
with sliding speed accordingly. They were fitted
by a curve with linear trend to calculate the average
slope of each curve corresponding to the d/dv
value. The comparisons of d/dv values from the
three consecutive runs for the fresh and aged ATFs
at 26 and 100 °C are shown in Figs. 9 and 10,
respectively. Also, the variation of the slope value
obtained from the repetition tests is given.
Fig. 7 Friction coefficient against sliding speed for three consecutive runs at 26 °C of the different fresh and aged ATFs: (a)
1ATF (fresh); (b) 1ATF (aged); (c) 2ATF (fresh); and (d) 2ATF (aged).
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Fig. 8 Friction coefficient against sliding speed for three consecutive runs at 100 °C of the different fresh and aged ATFs: (a)
1ATF (fresh); (b) 1ATF (aged); (c) 2ATF (fresh); and (d) 2ATF (aged).
Fig. 9 d/dv values obtained at consecutive runs at 26 °C for the ATFs: (a) fresh; (b) aged.
Fig. 10 d/dv values obtained at consecutive runs at 100 °C for the ATFs: (a) fresh; (b) aged.
3.2.1 Influence of ATF oxidation on wet clutch friction
According to Figs. 7 and 8, aged 1ATF at both
temperatures and 2ATF at 100 °C exhibited increase
of friction coefficients mainly at low sliding speeds,
which can be mainly attributed to additives depletion,
such as ZDDPs and friction modifiers as that reported
by Zhao et al. [26]. They reported that friction
modifiers have a considerable effect of reducing
friction coefficient at low speed as well as detergents
help to enhance the anti-shudder property of ATFs.
The oxidation of ATFs increased friction as that
reported by Berglund et al. [10] for a sintered bronze
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friction material. Comparing both ATFs, 1ATF exhibited
the highest increase in friction coefficients at both
temperatures by oxidation. 1ATF (fresh and aged)
had less concentration of ZDDPs than 2ATF as
seen in Fig. 5(b), so the friction increase exhibited by
2ATF (aged) could be ascribed to the low concentration
and depletion of ZDDPs and friction modifiers.
The increased friction in a wet clutch by the oil
oxidation may be considered as positive since it
increases the capacity of torque transmission of
wet clutch [1, 10]. However, it could be negative if
friction is only raised at low sliding speeds because
it can decrease the anti-shudder property. In Figs.
9 and 10, the change in d/dv value by oxidation of
the ATFs at 26 and 100 °C can be seen, respectively.
Comparing the first run results to discard surface
alteration in some extent, 2ATF exhibited a slope
decrease at both temperatures after being oxidized
meanwhile 1ATF only decreased at 100 °C due to
oxidation. 1ATF exhibited higher d/dv values than
2ATF after being aged. Also, 1ATF presented an
increase of friction coefficient with sliding speed
at 26 °C due to oxidation. It may be due to 1ATF
was less oxidized than 2ATF by the thermal ageing
process (1ATF producing less carbonyl constituents
than 2ATF), so oxidized products from ATFs, in
particular, carbonyl compounds could reduce the
anti-shudder behavior of ATFs.
3.2.2 Influence of sliding distance on wet clutch friction
In Figs. 7 and 8, the friction coefficients exhibited
difference between each run, meaning that surface
alteration produced by progressive sliding distance
can increase or decrease friction. Typical optical
images from the surface of the pin samples before
and after being tested for the three runs by using
both fresh and aged ATFs at 100 °C, are illustrated
in Figs. 11(a)–11(e), respectively. Figure 11(a) shows
the friction material´s surface before testing meanwhile
Figs. 11(b, c) and 11(d, e) illustrate the typical surface
alteration produced on the friction material samples
for the total sliding distance (1,904 m) using 1ATF
and 2ATF, respectively. It was found that the surface
damage generated on the samples was quite similar
at both temperatures (26 and 100 °C). The decrease
in porosity reduces the ATF porous diffusion in
the friction material composite [3, 7], promoting
the formation of larger ATF film thickness during
sliding [27]. According to Figs. 7(a, c) and 8(a, c),
both fresh ATFs exhibited a decrease of friction
coefficient at the different sliding speeds with
Fig. 11 Micrographs from typical surface of pin specimens before and after being tested using the different oils: (a) untried sample; (b) using fresh 1ATF; (c) using aged 1ATF; (d) using fresh 2ATF; and (e) using aged 2ATF.
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progressive sliding distance (2nd and 3rd runs) in
contrast to the results from the 1st run (shortest
sliding distance). So, progressive sliding distance
reduces friction coefficient in wet clutch using
fresh ATFs. It could be possible due to that sliding
motion enhances the C–OH groups´ adsorption on
the friction material wear track reducing friction
coefficient as found and reported in Refs. [26, 27].
C–OH groups came from the organic friction
modifier´s hydrophilic head group and could
contribute to favorable surface triboreactions [26].
In contrast, according to Figs. 7(b, d) and 8(b, d),
both aged ATFs exhibited increased friction coefficients
at the different sliding speeds with progressive
sliding distance (2nd and 3rd runs) contrary to the
results from the 1st run (shortest sliding distance).
Considering the oxidation caused to the ATFs by
thermal ageing, friction modifiers perhaps were
also depleted. In a similar way, these effects of
thermal ageing on the friction modifiers also
promoted variations in the dμ/dv value. According
to Figs. 9 and 10, fresh ATFs exhibited an increase
of dμ/dv value for the three runs at both temperatures
in contrast to aged ATFs, meaning that the surface
alteration (C–OH groups´ adsorption on the friction
material wear track) by the progressive sliding
using fresh ATFs could be beneficial for the anti-
shudder property in contrast to that alteration
formed by the aged oils, in which friction modifiers
are depleted.
The changes in surface roughness of the samples
can be a representation of the surface porosity changes
occurred. The comparison of the changes in surface
roughness of the friction material samples generated
by using both oils for the total sliding distance at
26 and 100 °C is shown in Figs. 12(a) and 12(b),
respectively, meanwhile the comparison of surface
roughness of the tested steel plates is depicted in
Fig. 13. The friction material samples and steel plates
exhibited a decrease of surface roughness for all
the cases. It suggests that porosity of friction material
samples decreased with sliding distance while surface
of steel plates was polished. It may be other reason
for the decrease in friction coefficients seen in Figs.
7(a–d) and 8(a, c). The sliding distance tested was
not enough to produce a significant glaze layer on
the friction material surfaces. So, longer distances
are further required to evaluate more advanced
stages of alteration of the wet clutch disks surfaces.
Holgerson [20] reported results of different
parameters measured during the engagement in a
wet clutch apparatus approaching actual conditions
of an AT. The tests were carried out with input
parameters that simulated an actual gear change
in an AT at 4,000 rev/min and 50% throttle. The
starting sliding speed in the wet clutch was 20.9 m/s,
the drive torque was 23.6 N·m with an inertia of
0.0081 kg·m2, the maximum pressure was 1.4 MPa
and mean pressure of 0.8 MPa for an engagement
time of 0.67 s. In Fig. 14, the mean linear trend
curve of the speed change against engagement
time obtained from those experiments is shown.
The sliding distance, s, in the engagement can be
calculated by:
0
dt
s v t (1)
where v is velocity and t is the engagement time.
Hence, the change of velocity with time was calculated
by:
v = 31.194 t + 20.9 (2)
Fig. 12 Comparison of surface roughness (Sa) measured by optical profilometer from the surfaces of the friction material
specimens tested at: (a) 26 °C; (b) 100 °C.
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Fig. 13 Comparison of surface roughness (Sa) measured by optical profilometer from the surfaces of the tested steel disks.
Fig. 14 Linear trend curve from the measured speed vs engagement time curve obtained in a wet clutch test rig.
So, the total sliding distance in a simulated wet
clutch engagement was found to be around 7 m.
Besides, considering that the average number of
wet clutch engagements for gear change from the
lowest to the highest in common passenger cars
for a finished distance of 100 km was reported to
be around 140 for highway driving [28], it can be
proposed that the approached amount of sliding
distance in a wet clutch for 100 km can be around
231 m under such conditions. Hence, the surface
alteration produced in the friction plates for the
sliding distances tested (0, 634, 1,269 m) could
represent the amounts of sliding distance occurred
in a wet clutch of a passenger car driven in highway
for 0, 275, and 550 km, respectively. These are very
short driven distances in comparison with the actual
service life of a wet clutch that is expected to be
over 200,000 km. Considering this approach and
the surface alteration observed in the friction samples,
it can be argued that the friction coefficient behavior
obtained from these tests corresponds to that behavior
exhibited in the running-in stage. The chemical
characterization of the tribo-film generated on the
surfaces and further glaze formation at much longer
sliding distances will be helpful for the understanding
of more severe wet clutch service conditions.
3.2.3 Influence of temperature on wet clutch friction
The friction coefficients generated by testing both
fresh and aged ATFs were lower at 100 °C than 26 °C,
suggesting that friction coefficients are reduced by
raising temperature. This reduction of friction with
the rise of temperature has been already reported
by other authors [10–12]. So, it is related to the
activation of additives (friction modifiers) at high
temperatures decreasing friction. This reduction of
friction may be negative for the anti-shudder property
of an ATF because friction is not decreased only at
low sliding speeds but also at higher speeds, which
cannot improve the slope value in the μ/v plots.
Comparing the Figs. 9 and 10, the slope value of
d/dv was decreased at 100 °C for both fresh and
aged ATFs. Hence, the increase of temperature can
be considered also as a parameter that affects the
anti-shudder property of the oils.
It is noteworthy that the results of measurement
and discussion of d/dv values obtained in this
work correspond to results obtained in a pin-on-
disk test set-up as a first evaluation, so this research
requires of SAE#II tests to certainly discuss on the
anti-shudder property of the oils, which will be
subject of future work.
4 Conclusions
1) Series of pin-on-disk tests were carried out to
determine the friction coefficients at different sliding
speeds of a wet clutch using two different
automatic transmission fluids (ATFs) and evaluating
the effects of oil oxidation, sliding distance, and
temperature in the running-in stage.
2) The ATFs presented changes in kinematic
viscosity and viscosity index due to oxidation.
However, one of the ATFs tested (2ATF) was
considerably more oxidized than the other (1ATF)
by the same thermal ageing process, so it presented
higher increase in viscosity and decrease in viscosity
index.
3) The friction coefficients of the wet clutch were
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increased by the ATFs oxidation meanwhile the
d/dv values were decreased in most cases. It
suggested that ATF oxidation could enhance the
torque capacity of the wet clutch but reduce the
ATF anti-shudder property.
4) Alteration of the surface of friction plates was
observed to increase with sliding distance after
each run conducted exhibiting a decrease in surface
roughness and increase of dark regions on the
sample´s surface. The sliding distance progression
generated decrease of friction coefficients in most
cases. However, fresh ATFs exhibited an increase of
d/dv value for the three runs at both temperatures
in contrast to aged ATFs, meaning that fresh ATFs
may promote a positive influence in the anti-shudder
property at advanced sliding distances in contrast
to oxidized ATFs.
5) The sliding distances tested were not enough
to produce significant glazing on the friction
material specimens, so the results were found to
meet wet clutch running-in conditions.
Acknowledgements
The authors would like to acknowledge to CNMN-
IPN for the support in the spectroscopic analyses
of our oil samples. We also thank to “Laboratorio
de Reología y Física de la Materia Blanda” from
ESFM-Instituto Politécnico Nacional for the assistance
and equipment support for the viscosity measurements
of our oil samples.
Open Access This article is licensed under a Creative
Commons Attribution 4.0 International License,
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exceeds the permitted use, you will need to obtain
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Leonardo Israel FARFAN CABRERA.
He received his M.S. and Ph.D.
degrees in mechanical engineering
from Instituto Politécnico Nacional,
Mexico, in 2014 and 2018, respectively.
He joined the School of Engineering and Sciences
and the Nanotechnology research group at Tecnologico
de Monterrey since 2018. His current position is
assistant professor. His research areas cover the
tribology of automotive components and engineering
polymers, bio-lubricants, and tribo-testing.
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